AIX reports the CODESET as "ISO8859-1" in the POSIX locale.

AIX reports the CODESET as "ISO8859-1" in the POSIX locale.
Treating that as a safe encoding is OK because even when other
systems return that string for real ISO8859-1, it is still
safe in the sense that it is ASCII-compatible and stateless.

Issue reported by Val dot Baranov at duke dot edu.  Additional
information provided by Michael dot Felt at felt dot demon dot nl.
Tested by Michael Felt on AIX 6.1 and by Val Baranov on AIX 7.1.
Tweak and OK djm@.